DESCRIPTION (Applicant's Abstract): The long-term objectives of this application are to elucidate the mechanisms that control phenotypic expression of the alveolar epithelium. The alveolar epithelium comprises two morphologically distinct differentiated epithelial cells, type I and type II cells, both of which are thought to be critical for normal lung function. Although the establishment and maintenance of normal alveolar epithelium is essential for mammalian life, factors controlling the expression of cellular phenotype are not well understood. In vivo, type II cells have the capacity to repair injured alveoli, acquiring at least some characteristics of the type I cell phenotype. The process by which one "stable" cellular phenotype changes into another phenotype has been termed "transdifferentiation." From in vitro studies with type II cells, it has been proposed that the transdifferentiation from the type II to the type I cell phenotype is reversible. Taken together, the observations in vivo and vitro have generated the following hypotheses: 1)transdifferentiation can occur bi-directionally between type II and type I cells; 2)there is inverse co-regulation of expression of each phenotype. Lack of suitable cell-culture models and sufficient numbers of cell-specific biochemical and molecular markers has made it difficult to study regulation of phenotypic expression in the lung. Although progress has been made in the development of appropriate markers and in vitro systems, there are still only a small number of cell-specific markers. Another significant problem has been the lack of methodology for isolating and culturing type I cells. We have recently developed methods for isolating, culturing, and transfecting type I cells, permitting direct study of this cell type in vitro. In the current application, we propose studies comparing freshly isolated (type I and type II) cells to cells cultured under conditions that favor expression of one or the other phenotype to achieve four goals: 1) to define functional and molecular characteristics of each phenotype in greater detail; 2) to identify additional specific markers for each phenotype; 3) to determine the extent to which transdifferentiation occurs in vitro; 4) and to identify and test candidate genes for regulating transdifferentiation. The results of such studies have potential applications to clinical problems. Because the recovery from acute lung injury is dependent on the regeneration of a normal alveolar epithelium, a greater understanding of how alveolar epithelial phenotypic expression is regulated may lead to the development of new testable hypotheses regarding treatments for acute lung injury.